Soft magnetic alloy with ultrafine crystal grains and method of producing same

ABSTRACT

A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula: 
     
         Co.sub.100.sub.-x-y-z-a-b Fe.sub.a M.sub.x B.sub.y X.sub.z T.sub.b (atomic 
    
      %) 
     wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, X represents at least one element selected from Si, Ge, P, Ga, Al and N, T represents at least one element selected from Cu, Ag, Au, platinum group elements, Ni, Sn, Be, Mg, Ca, Sr and Ba, 0&lt;a≦30, 2≦x≦15, 10≦y≦25, 0≦z≦10, 0&lt;b≦10, and 12&lt;x+y+z+b≦35. Such a magnetic alloy can be produced by producing an amorphous alloy having the above composition, and subjecting the resulting amorphous alloy to a heat treatment to cause crystallization, thereby providing the resulting alloy having a structure, at least 50% of which is occupied by crystal grains having an average grain size of 500 Å or less.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic alloy with ultrafine crystal grains excellent in magnetic properties and their stability, a major part of the alloy structure being occupied by ultrafine crystal grains, suitable for magnetic cores for transformers, choke coils, etc.

Conventionally used as core materials for magnetic cores such as choke coils are ferrites, silicon steels, amorphous alloys, etc. showing relatively good frequency characteristics with small eddy current losses.

However, ferrites show low saturation magnetic flux densities and their permeabilities are relatively low if the frequency characteristics of their permeabilities are flat up to a high-frequency region. On the other hand, for those showing high permeabilities in a low frequency region, their permeabilities start to decrease at a relatively low frequency. With respect to Fe--Si--B amorphous alloys and silicon steels, they are poor in corrosion resistance and high-frequency magnetic properties.

In the case of Co-base amorphous alloys, their magnetic properties vary widely with time, suffering from low reliability.

In view of these problems, various attempts have been made. For instance, Japanese Patent Laid-Open No. 64-73041 discloses a Co--Fe--B alloy having a high saturation magnetic flux density and a high permeability. However, it has been found that this alloy is poor in heat resistance and stability of magnetic properties with time.

OBJECT AND SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a magnetic alloy having high permeability and a low core loss required for magnetic parts such as choke coils, the stability of these properties being stable with time, and further showing excellent heat resistance and corrosion resistance.

As a result of intense research in view of the above object, the inventors have found that in the Co--Fe--B crystalline alloys, by increasing the amount of B than that described in Japanese Patent Laid-Open No. 64-73041 and adding a transition metal selected from Nb, Ta, Zr, Hf, etc. to the alloys, the alloys have ultrafine crystal structures, thereby solving the above-mentioned problems. The present invention has been made based upon this finding.

Thus, the magnetic alloy with ultrafine crystal grains according to the present invention has a composition represented by the general formula:

    Co.sub.100-x-y M.sub.x B.sub.y (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, 2≦x≦15, 10<y≦25, and 12<x+y≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 500 Å or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an X-ray diffraction pattern of the alloy of the present invention before heat treatment;

FIG. 2 is a graph showing an X-ray diffraction pattern of the alloy of the present invention heat-treated at 700° C.;

FIG. 3 is a graph showing the relation between effective permeability and heat treatment temperature;

FIG. 4 is a graph showing the relation between a heat treatment temperature and saturation magnetostriction; and

FIG. 5 is a graph showing the relation between a core loss and frequency with respect to the alloy of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the above magnetic alloy of the present invention, B is an indispensable element, effective for making the crystal grains ultrafine and controlling the alloy's magnetostriction and magnetic anisotropy.

M is at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, which is also an indispensable element.

By the addition of both M and B, the crystal grains can be made ultrafine.

The M content (x), the B content (y) and the total content of M and B (x+y) should meet the following requirements:

    2≦x≦15.

    10<y≦25.

    12<x+y≦35.

When x and y are lower than the above lower limits, the alloy has poor soft magnetic properties and heat resistance. On the other hand, when x and y are larger than the above upper limits, the alloy has poor saturation magnetic flux density and soft magnetic properties. Particularly, the preferred ranges of x and y are:

    5≦x≦15.

    10<y≦20.

    12<x+y≦30.

With these ranges, the alloys show excellent high-frequency soft magnetic properties and heat resistance.

According to another aspect of the present invention, the above composition may further contain either one or two components selected from Fe, at least one element (X) selected from Si, Ge, P, Ga, Al and N, at least one element (T) selected from Cu, Ag, Au, platinum group elements, Ni, Sn, Be, Mg, Ca, Sr and Ba.

Accordingly, the following alloys are also included in the present application.

    Co.sub.100-a-x-y Fe.sub.a M.sub.x B.sub.y (atomic %)       (1)

wherein 0<a≦30, 2≦x≦15, 10<y≦25, and 12<x+y≦35.

    Co.sub.100-x-y-x M.sub.x B.sub.y X.sub.z (atomic %)        (2)

wherein 2≦x≦15, 10<y≦25, 0<z≦10, and 12<x+y+z≦35.

    Co.sub.100-x-y-b M.sub.x B.sub.y T.sub.b (atomic %)        (3)

wherein 2≦x≦15, 10<y≦25, 0<b≦10, and 12<x+y+b≦35.

    Co.sub.100-a-x-y-z Fe.sub.a M.sub.x B.sub.y X.sub.z (atomic %)(4)

wherein 0<a≦30, 2≦x≦15, 10<y≦25, 0<z≦10, and 12<x+y+z≦35

    Co.sub.100-x-y-a-b Fe.sub.a M.sub.x B.sub.y T.sub.b (atomic %)(5)

wherein 0<a≦30, 2≦x≦15, 10<y≦25, 0<b≦10, and 12<x+y+b≦35.

    Co.sub.100-x-y-z-b M.sub.x B.sub.y X.sub.z T.sub.b (atomic %)(6)

wherein 2≦x≦15, 10<y≦25, 0<z≦10, 0<b≦10, and 12<x+y+z+b≦35.

    Co.sub.100-x-y-z-a-b Fe.sub.a M.sub.x B.sub.y X.sub.z T.sub.b (atomic %)(7)

wherein 0<a≦30, 2≦x≦15, 10<y≦25, 0<z≦10, 0<b≦10, and 12<x+y+z+b≦35.

With respect to Fe, it may be contained in an amount of 30 atomic % or less, to improve permeability.

With respect to the element X, it is effective to control magnetostriction and magnetic anisotropy, and it may be added in an amount of 10 atomic % or less. When the amount of the element X exceeds 10 atomic %, the deterioration of saturation magnetic flux density, soft magnetic properties and heat resistance takes place.

With respect to the element T, it is effective to improve corrosion resistance and to control magnetic properties. The amount T (b) is preferably 10 atomic % or less. When it exceeds 10 atomic %, extreme decrease in saturation magnetic flux density takes place.

Each of the above-mentioned alloys of the present invention has a structure based on Co crystal grains with B compounds. The crystal grains have an average grain size of 500 Å or less. Particularly when the average grain size is 200 Å or less, excellent soft magnetic properties can be obtained.

The reason why excellent soft magnetic properties can be obtained in the magnetic alloy with ultrafine crystal grains of the present invention are considered as follows: In the present invention, M and B form ultrafine compounds uniformly dispersed in the alloy structure by a heat treatment, suppressing the growth of Co crystal grains. Accordingly, the magnetic anisotropy is apparently offset by this action of making the crystal grains ultrafine, resulting in excellent soft magnetic properties.

In the present invention, ultrafine crystal grains should be at least 50% of the alloy structure, because if otherwise, excellent soft magnetic properties would not be obtained.

According to a further aspect of the present invention, there is provided a method of producing a magnetic alloy with ultrafine crystal grains comprising the steps of producing an amorphous alloy having either one of the above-mentioned compositions, and subjecting the resulting amorphous alloy to a heat treatment to cause crystallization, thereby providing the resulting alloy having a structure, at least 50% of which is occupied by crystal grains having an average grain size of 500 Å or less.

Depending upon the heat treatment conditions, an amorphous phase may remain partially, or the alloy structure may become 100% crystalline. In either case, excellent soft magnetic properties can be obtained.

The amorphous alloy is usually produced by a liquid quenching method such as a single roll method, a double roll method, a rotating liquid spinning method, an atomizing method, etc. The amorphous alloy is subjected to heat treatment in an inert gas atmosphere, in hydrogen or in vacuum to cause crystallization, so that at least 50% of the alloy structure is occupied by crystal grains having an average grain size of 500 Å or less. In the process of crystallization, the B compounds, contributing to the generation of an ultrafine structure. The B compounds formed appear to be compounds of B and M elements (at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn).

The heat treatment according to the present invention is usually conducted at 450° C.-800° C., which means that an extremely high temperature can be employed in this heat treatment. The alloy of the present invention can be subjected to a heat treatment in a magnetic field. When a magnetic field is applied in one direction, magnetic anisotropy in one direction can be generated.

By conducting the heat treatment in a rotating magnetic field, further improvement in soft magnetic properties can be achieved. In addition, the heat treatment for crystallization can be followed by a heat treatment in a magnetic field. Incidentally, by increasing the temperature of a roll, and controlling the cooling conditions, the alloy of the present invention can be produced directly without passing through a state of an amorphous alloy.

The present invention will be explained in further detail by way of the following Examples, without intending to restrict the scope of the present invention.

EXAMPLE 1

An alloy melt having a composition (atomic %) of 7% Nb, 22% B and substantially balance Co was rapidly quenched by a single roll method to produce a thin amorphous alloy ribbon of 5 mm in width and 12 μm in thickness.

The X-ray diffraction pattern of this amorphous alloy before a heat treatment is shown in FIG. 1.

It is clear from FIG. 1 that this pattern is a halo pattern peculiar to an amorphous alloy. This alloy had an crystallization temperature of 480° C. Next, this thin alloy ribbon was formed into a toroidal core of 19 mm in outer diameter and 15 mm in inner diameter, and this core was subjected to a heat treatment at 400° C.-700° C. in an Ar gas atmosphere to cause crystallization.

The X-ray diffraction pattern of the alloy obtained by the heat treatment at 700° C. is shown in FIG. 2. As a result of X-ray diffraction analysis and transmission electron photomicrography, it was confirmed that the alloy after a 700° C. heat treatment had a structure, almost 95% of which is constituted by ultrafine crystal grains made of Co and B compounds and having an average grain size of 80 Å.

FIG. 3 shows the dependency of effective permeability μ_(e) at 1 kHz on a heat treatment temperature, and FIG. 4 shows the dependency of saturation magnetostriction λ_(s) on a heat treatment temperature. In either case, the heat treatment was conducted at various temperatures for 1 hour without applying a magnetic field.

It is clear from FIGS. 3 and 4 that even at a high heat treatment temperature exceeding the crystallization temperature, good soft magnetic properties can be obtained, and that their levels are comparable to those of amorphous alloys. With respect to saturation magnetostriction, it increases from a negative value in an amorphous state to larger than 0 when the heat treatment temperature exceeds the crystallization temperature, and becomes a positive value of about +1×10⁻⁸ at 700° C. Thus, it is confirmed that the alloy of the present invention shows low magnetostriction.

Next, with respect to a wound core constituted by an amorphous alloy heat-treated at 400° C. and a wound core constituted by a crystalline alloy obtained by a heat treatment at 700° C., they were kept at 120° C. for 1000 hours to measure their effective permeability μ_(e) at 1 kHz. As a result, it was observed that the effective permeability μ_(e) was reduced to 80% of the initial level in the case of the amorphous alloy, while it was reduced only to 97% of the initial value in the case of the alloy of the present invention. Thus, it was confirmed that the alloy of the present invention suffers from only slight change of effective permeability with time.

EXAMPLE 2

Thin amorphous alloy ribbons of 5 mm in width and 18 μm in thickness having the compositions shown in Table 1 were produced by a single roll method. Next, each of these thin alloy ribbons was formed into a toroidal core of 19 mm in outer diameter and 15 mm in inner diameter, and subjected to a heat treatment at 550° C.-800° C. in an Ar gas atmosphere to cause crystallization.

As a result of X-ray diffraction analysis and transmission electron photomicrography, it was confirmed that the alloys after the heat treatment had structures mostly constituted by ultrafine crystal grains made of Co and B compounds and having an average grain size of 500 Å or less. The details are shown in Table 1.

With respect to the magnetic cores after the heat treatment, core loss Pc at f=100 kHz and Bm=2 kG, and an effective permeability (μ_(elk)) at 1 kHz were measured. The results are shown in Table 1. The magnetic cores were also kept in a furnace at 600° C. for 30 minutes, and then cooled to room temperature to measure core loss Pc'. The ratios of Pc'/Pc are also shown in Table 1.

Further, thin alloy ribbons subjected to heat treatment were immersed in tap water for 1 week to evaluate corrosion resistance. Results are shown in Table 1, in which ○ represents alloys having substantially no rust, Δ represents those having slight rust, and x represents those having large rusts. Effective permeability μ_(elk) (24) at 1 kHz after keeping at 120° C. for 24 hours was measured. The values of μ_(elk) (24)/μ_(elk) are shown in Table 1.

It is clear from Table 1 that the alloys of the present invention show extremely high permeability, low core loss and excellent corrosion resistance. Accordingly, they are suitable as magnetic core materials for transformers, chokes, etc. Further, since their Pc'/Pc is nearly 1, their excellent heat resistance is confirmed, and since their μ_(elk) (24)/μ_(elk) is near 1, it is confirmed that the change of magnetic properties with time is small. Thus, the alloys of the present invention are suitable for practical applications.

                                      TABLE 1                                      __________________________________________________________________________                   Average                                                                             Crystal                                                                   Grain                                                                               Grain                                                       Sample                                                                             Composition                                                                              Size Content                                                                             Pc       Corrosion  μ.sub.e1k (24)/                 No.*                                                                               (atomic %)                                                                               (Å)                                                                             (%)  (mW/cc)                                                                             μ.sub.e1k                                                                       Resistance**                                                                          Pc'/Pc                                                                             μ.sub. e1k                      __________________________________________________________________________     1   Co.sub.bal Zr.sub.7 B.sub.22                                                             50   80   520  9100                                                                               ∘                                                                         1.02                                                                               0.99                               2   Co.sub.bal Hf.sub.7 B.sub.22                                                             60   90   530  8800                                                                               ∘                                                                         1.03                                                                               0.98                               3   Co.sub.bal Ta.sub.8 B.sub.19                                                             50   almost                                                                              460  9600                                                                               ∘                                                                         1.02                                                                               1.00                                                  100                                                         4   Co.sub.bal Nb.sub.8 B.sub.23                                                             40   90   440  7200                                                                               ∘                                                                         1.01                                                                               1.01                               5   Co.sub.bal Fe.sub.5 Hf.sub.8 Mn.sub.0.8                                                  55   79   470  7900                                                                               ∘                                                                         0.99                                                                               0.97                                   B.sub.19 Ga.sub.0.5                                                        6   Co.sub.bal Fe.sub.6 Ni.sub.2 Zr.sub.9 B.sub.20                                           56   90   480  7700                                                                               ∘                                                                         1.01                                                                               0.98                                   Al.sub.1                                                                   7   Co.sub.bal Ti.sub.10 B.sub.22 Ga.sub.0.8                                                 75   95   510  8200                                                                               ∘                                                                         1.04                                                                               1.00                               8   Co.sub.bal Zr.sub.13 B.sub. 20 P.sub.0.7 Cu.sub.1                                        40   80   520  8500                                                                               ∘                                                                         1.02                                                                               0.99                               9   Co.sub.bal Hf.sub.10 B.sub.22 Si.sub.1 Ru.sub.2                                          55   90   440  8200                                                                               ∘                                                                         1.03                                                                               0.98                               10  Co.sub.bal Fe.sub.8 Nb.sub.8 B.sub.19 Ge.sub.1                                           80   75   480  7200                                                                               ∘                                                                         0.99                                                                               0.99                                   Ni.sub.1                                                                   11  Co.sub.bal Zr.sub.8 B.sub.24 Be.sub.0.5                                                  70   90   460  6800                                                                               ∘                                                                         1.01                                                                               0.97                                   Rh.sub.2                                                                   12  Co.sub.bal Fe.sub.4.7 Si.sub.15 B.sub.10                                                 --   --   --   8500                                                                               ∘                                                                         36.8                                                                               0.62                                   Amorphous                                                                  13  Fe.sub.bal Al.sub.7.6 Si.sub.17.9                                                        --   --   --   10000                                                                              Δ                                                                               1.11                                                                               1.00                               14  Fe.sub.bal Si.sub.12.5                                                                   --   --   --   2800                                                                               x      1.21                                                                               0.99                               __________________________________________________________________________      Note                                                                           *: Sample Nos. 1-11: Present invention.                                        Sample Nos. 12-14: Conventional alloy.                                         **: Corrosion resistance                                                       ∘: Good.                                                           Δ: Fair.                                                                 x: Poor.                                                                 

EXAMPLE 3

An alloy melt having a composition (atomic %) of 7% Nb, 2% Ta. 5% Fe, 23% B and balance substantially Co was rapidly quenched by a single roll method in a helium gas atmosphere at a reduced pressure to produce a thin amorphous alloy ribbon of 6 μm in thickness. Next, this thin amorphous alloy ribbon was coated with MgO powder in a thickness of 0.5 μm by an electrophoresis method and then wound to a toroidal core of 15 mm in outer diameter and 13 mm in inner diameter. This core was subjected to a heat treatment in an argon gas atmosphere while applying a magnetic field in a direction parallel to the width of the thin ribbon. It was kept at 700° C. in a magnetic field of 4000 Oe, and then cooled at about 5° C./min. The heat-treated alloy was crystalline, having a crystalline structure substantially 100% composed of ultrafine crystal grains having an average grain size of 90 Å.

FIG. 5 shows the frequency characteristics of core loss at B_(m) =2 kG with respect to the heat-treated magnetic core (A) of the present invention. For comparison, a magnetic core (B) made of Mn-Zn ferrite is also shown.

It is clear from FIG. 5 that the alloy of the present invention shows low core loss, meaning that it is promising for high-frequency transformers, etc.

EXAMPLE 4

An amorphous alloy layer of 3 μm in thickness having a composition (atomic %) of 7.2% Nb, 18.8% B and balance substantially Co was formed on a fotoceram substrate by an RF sputtering apparatus. In an X-ray diffraction analysis, the layer showed a halo pattern peculiar to an amorphous alloy. This amorphous alloy layer was heated at 650° C. for 1 hour in a nitrogen gas atmosphere and then cooled to room temperature to measure X-ray diffraction. As a result, Co crystal peaks and slight NbB compound phase peaks were observed. As a result of transmission electron photomicrography, it was confirmed that substantially 100% of the alloy structure was occupied by ultrafine crystal grains having an average grain size of 90 Å.

Next, this layer was measured with respect to effective permeability μ_(elM) at 1 MHz by an LCR meter. Thus, it was found that μ_(elM) was 2200. The details are shown in Table 2.

EXAMPLE 5

Alloy layers having compositions shown in Table 2 were produced on fotoceram substrates in the same manner as in Example 4. Their saturation magnetic flux densities B₁₀ were measured by a vibration-type magnetometer, and their effective permeabilities μ_(elM) at 1 MHz were measured by an LCR meter. The results are shown in Table 2. Incidentally, any heat-treated alloy had an ultrafine crystalline structure having an average grain size of 500 Å or less. The details are shown in Table 2.

Since the alloys of the present invention showed as high saturation magnetic flux densities and μ_(elM) as those of Fe--Si--Al alloys, the alloys of the present invention are suitable for magnetic heads.

                                      TABLE 2                                      __________________________________________________________________________                    Average                                                                             Crystal                                                                   Grain                                                                               Grain                                                      Sample                                                                             Composition                                                                               Size Content  Phase                                             No.*                                                                               (atomic %) (Å)                                                                             (%)  μ.sub.e1M                                                                       Structure                                         __________________________________________________________________________     15  Co.sub.bal Zr.sub.8.2 B.sub.11.5                                                          140  90   2900                                                                               Co + Zr - B                                                                    Compound                                          16  Co.sub.bal Hf.sub.7.5 B.sub.12.4                                                           90  80   2700                                                                               Co + Hf - B                                                                    Compound                                          17  Co.sub.bal Ta.sub.7.8 B.sub.15.1                                                           70  70   2500                                                                               Co + Ta - B                                                                    Compound                                          18  Co.sub.bal Nb.sub.8.2 B.sub.13.2                                                           80  90   1800                                                                               Co + Nb - B                                                                    Compound                                          19  Co.sub.bal Cr.sub.12.1 B.sub.13.2 Si.sub.0.9                                              200  90   1100                                                                               Co + Cr - B                                                                    Compound                                          20  Co.sub.bal W.sub.8.5 B.sub.14.3 Ge.sub.1.2                                                 60  90   1300                                                                               Co + W - B                                                                     Compound                                          21  Co.sub.bal Hf.sub.8.3 B.sub.12.9 Ga.sub.1.1                                                 90 80   1700                                                                               Co + Hf - B                                                                    Compound                                          22  Co.sub.bal Zr.sub.8.5 B.sub.15.9 Al.sub.1.2                                                65  almost                                                                              1800                                                                               Co + Zr - B                                                           100      Compound                                          23  Co.sub.bal Nb.sub.8.7 B.sub.14.8 N.sub.0.3                                                 50  85   1100                                                                               Co + Nb - B                                                                    Compound                                          24  Co.sub.bal Mo.sub.12.0 B.sub.16.8 Al.sub.1.4                                              130  80   1200                                                                               Co + Mo - B                                                                    Compound                                          25  Co.sub.bal Ti.sub.10.5 B.sub.18.1 Ga.sub.1.3                                              120  90   1100                                                                               Co + Ti - B                                                                    Compound                                          26  Co.sub.bal Zr.sub.12.7 B.sub.17.3 P.sub.1.2                                                40  90   1000                                                                               Co + Zr - B                                                                    Compound                                          27  Co.sub.bal Hf.sub.9.7 B.sub.14.3 Si.sub.1.1                                                80  75   1800                                                                               Co + Hf - B                                                                    Compound                                          28  Co.sub.bal Nb.sub.7.7 B.sub.11.8 Ge.sub.1.1                                                60  95   1000                                                                               Co + Nb - B                                                                    Compound                                          29  Co.sub.bal Ti.sub.13.8 B.sub.12.2 Sn.sub.1.8                                               70  almost                                                                              1100                                                                               Co + Ti - B                                                           100      Compound                                          30  Co.sub.bal Zr.sub.10.1 B.sub.12.6 Be.sub.1.3                                               65  95   1800                                                                               Co + Zr - B                                                                    Compound                                          31  Fe.sub.bal Al.sub.7.6 Si.sub.17.9                                                         1000 100  1500                                                                               bcc Fe                                            32  Fe.sub.bal Si.sub.12.5                                                                    1500 100   400                                                                               bcc Fe                                            33  Co.sub.bal Nb.sub.13.0 Zr.sub.3.0                                                         --   --   3500                                                                               Amorphous                                             Amorphous                                                                  __________________________________________________________________________      Note                                                                           *: Sample Nos. 15-30: Present invention.                                       Sample Nos. 31-33: Conventional alloy.                                   

EXAMPLE 6

Thin amorphous alloy ribbons of 5 mm in width and 15 μm in thickness having compositions shown in Table 3 were produced by a single roll method. Next, each of these thin alloy ribbons was formed into a toroidal core of 19 mm in outer diameter and 15 mm in inner diameter, and subjected to a heat treatment at 550° C.-700° C. in an Ar gas atmosphere to cause crystallization.

As a result of X-ray diffraction analysis and transmission electron photomicrography, it was confirmed that the alloys after the heat treatment had structures mostly constituted by ultrafine crystal grains made of Co and B compounds and having an average grain size of 500 Å or less. The details are shown in Table 3.

                                      TABLE 3                                      __________________________________________________________________________                     Average                                                                             Crystal                                                                   Grain                                                                               Grain                                                     Sample                                                                             Composition Size Content  Phase                                            No.*                                                                               (atomic %)  (Å)                                                                             (%)  μ.sub.e1M                                                                       Structure                                        __________________________________________________________________________     34  Co.sub.bal Zr.sub.8 B.sub.12                                                               80   almost                                                                              3300                                                                               Co + Zr - B                                                           100      Compound                                         35  Co.sub.bal Hf.sub.7 B.sub.12                                                               90   almost                                                                              3600                                                                               Co + Hf - B                                                           100      Compound                                         36  Co.sub.bal Ta.sub.8 B.sub.15                                                               60    90  3200                                                                               Co + Ta - B                                                                    Compound                                         37  Co.sub.bal Nb.sub.8 B.sub.13                                                               50   almost                                                                              2600                                                                               Co + Nb - B                                                           100      Compound                                         38  Co.sub.bal Hf.sub.8 Mn.sub.0.6 B.sub.13 Ga.sub.1                                           80    95  2800                                                                               Co + Hf - B                                                                    Compound                                         39  Co.sub.bal Zr.sub.9 B.sub.16 Al.sub.1                                                      60    85  2200                                                                               Co + Zr - B                                                                    Compound                                         40  Co.sub.bal Ti.sub.11 B.sub.18 Ga.sub.0.5                                                   70    90  2300                                                                               Co + Ti - B                                                                    Compound                                         41  Co.sub.bal Zr.sub.13 B.sub.17 P.sub.0.5 Cu.sub.1                                           50   almost                                                                              2400                                                                               Co + Zr - B                                                           100      Compound                                         42  Co.sub.bal Hf.sub.10 B.sub.14 Si.sub.1 Ru.sub.1 Cu.sub.5                                   60   almost                                                                              2500                                                                               Co + Hf - B                                                           100      Compound                                         43  Co.sub.bal Nb.sub.8 B.sub.11 Ge.sub.1 Ni.sub.1                                             80   almost                                                                              2800                                                                               Co + Nb - B                                                           100      Compound                                         44  Co.sub.bal Zr.sub.10 B.sub.13 Be.sub.0.5 Rh.sub.1                                          70   almost                                                                              2300                                                                               Co + Zr - B                                                           100      Compound                                         45  Co.sub.bal Nb.sub.13 Zr.sub.3                                                              --   --   2300                                                                               Amorphous                                            Amorphous                                                                  46  Fe.sub.bal Al.sub.7.6 Si.sub.17.9                                                          --   --   1500                                                                               bcc Fe                                           47  Fe.sub.bal Si.sub.12.5                                                                     --   --    400                                                                               bcc Fe                                           __________________________________________________________________________      Note                                                                           *: Sample Nos. 34-44: Present invention.                                       Sample Nos. 45-47: Conventional alloy.                                   

EXAMPLE 7

Alloy layers having compositions shown in Table 4 were produced on fotoceram substrates in the same manner as in Example 4, and subjected to a heat treatment at 650° C. for 1 hour to cause crystallization. The average grain size and the percentage of crystal grains of each heat-treated alloy are shown in Table 4. At this stage, their μ_(elMO) was measured. Next, these alloys were introduced into an oven at 600° C., and kept for 30 minutes and cooled to room temperature to measure their μ_(elM'). Their μ_(elM') /μ_(elMO) ratios are shown in Table 4.

The alloy layers of the present invention show μ_(elM') /μ_(elMO) close to 1, and suffer from little deterioration of magnetic properties even at a high temperature, showing good heat resistance. On the other hand, the conventional Co--Fe--B alloy and the amorphous alloy show μ_(elM') /μ_(elMO) much smaller than 1, meaning that their magnetic properties are deteriorated. Thus, the alloys of the present invention are suitable for producing high-reliability magnetic heads.

                                      TABLE 4                                      __________________________________________________________________________                    Average                                                                             Crystal                                                                   Grain                                                                               Grain                                                      Sample                                                                             Composition                                                                               Size Content                                                                             μ.sub.e1M' /                                                                    Phase                                             No.*                                                                               (atomic %) (Å)                                                                             (%)  μ.sub.e1M0                                                                      Structure                                         __________________________________________________________________________     48  Co.sub.bal Fe.sub.15.1 Zr.sub.8.6 B.sub.17.2                                              130  almost                                                                              0.96                                                                               Co + Zr - B                                                           100      Compound                                          49  Co.sub.bal Hf.sub.8.7 B.sub.10.5                                                          120  almost                                                                              0.95                                                                               Co + Hf - B                                                           100      Compound                                          50  Co.sub.bal Fe.sub.0.2 Ta.sub.7.7 B.sub.11.2                                               110   95  0.94                                                                               Co + Ta - B                                                                    Compound                                          51  Co.sub.bal Nb.sub.8.3 B.sub.22.5                                                           90  almost                                                                              0.92                                                                               Co + Nb - B                                                           100      Compound                                          52  Co.sub.bal Cr.sub.12.2 B.sub.25.1 Si.sub.0.6                                              460  almost                                                                              0.90                                                                               Co + Cr - B                                                           100      Compound                                          53  Co.sub.bal W.sub.8.9 B.sub.14.4 Ge.sub.1.4                                                130   90  0.91                                                                               Co + W - B                                                                     Compound                                          54  Co.sub.bal Mn.sub.12.4 B.sub.12.2 Ga.sub.1.1                                              440  almost                                                                              0.92                                                                               Co + Mn - B                                                           100      Compound                                          55  Co.sub.bal Hf.sub.8.3 B.sub.12.2 Ga.sub.1.1                                                70   95  0.91                                                                               Co + Hf - B                                                                    Compound                                          56  Co.sub.bal Zr.sub.8.6 B.sub.16.9 Al.sub.1.5                                                90   90  0.87                                                                               Co + Zr - B                                                                    Compound                                          57  Co.sub.bal Nb.sub.8.9 B.sub.15.9 N.sub.0.8                                                 80  almost                                                                              0.88                                                                               Co + Nb - B                                                           100      Compound                                          58  Co.sub.bal Mo.sub.12.1 B.sub.16.9 Al.sub.1.2                                              230  almost                                                                              0.98                                                                               Co + Mo - B                                                           100      Compound                                          59  Co.sub.bal Fe.sub.12.2 Ti.sub.10.5 B.sub.18.1                                             140   95  0.91                                                                               Co + Ti - B                                                                    Compound                                          60  Co.sub.bal Zr.sub.13.7 B.sub.17.4 P.sub.2.2                                                80   90  0.90                                                                               Co + Zr - B                                                                    Compound                                          61  Co.sub.bal Hf.sub.9.6 B.sub.14.2 Si.sub.1.2                                               160   85  0.88                                                                               Co + Hf -  B                                                                   Compound                                          62  Co.sub.bal Fe.sub.8.8 Ta.sub.8.2 B.sub.12.2                                                70   95  0.90                                                                               Co + Ta - B                                                                    Compound                                          63  Co.sub.bal Fe.sub.12 Ti.sub.13.8 B.sub.11.6                                               120   95  0.87                                                                               Co + Ti - B                                                                    Compound                                          64  Co.sub.bal Fe.sub.12 Ti.sub.13.8 B.sub.12.2                                                90  almost                                                                              0.89                                                                               Co + Ti - B                                                           100      Compound                                          65  Co.sub.bal Zr.sub.10.3 B.sub.12.8 Be.sub.0.4                                               80  almost                                                                              0.90                                                                               Co + Zr - B                                                           100      Compound                                          66  Co.sub.bal Fe.sub.6 B.sub.6 Si.sub.2                                                      --   --   0.12                                                                               fcc Fe                                            67  Co.sub.bal Nb.sub.13.0 Zr.sub.4                                                           --   --   0.12                                                                               Amorphous                                         __________________________________________________________________________

According to the present invention, magnetic alloys with ultrafine crystal grains having excellent permeability, corrosion resistance, heat resistance and stability of magnetic properties with time and low core loss can be produced. 

What is claimed is:
 1. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-x-y M.sub.x B.sub.y (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, 2≦x≦15, 10<y≦25, and 12<x+y≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 2. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-a-x-y Fe.sub.a M.sub.x B.sub.y (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, 0<a≦30, 2≦x≦15, 10<y≦25, and 12<x+y≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 3. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-x-y-z M.sub.x B.sub.y X.sub.z (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, X represents at least one element selected from Si, Ge, P, Ga, Al and N, 2≦x≦15, 10<y≦25, 0<z≦10, and 12<x+y+z≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 4. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-x-y-b M.sub.x B.sub.y T.sub.b (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, T represents at least one element selected from Cu, Ag, Au, platinum group elements, Ni, Sn, Be, Mg, Ca, Sr and Ba, 2≦x≦15, 10<y≦25, 0<b≦10, and 12<x+y+b≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 5. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-a-x-y-z Fe.sub.a M.sub.x B.sub.y X.sub.z (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, X represents at least one element selected from Si, Ge, P, Ga, Al and N, 0<a≦30, 2≦x≦15, 10<y≦25, 0<z≦10, and 12<x+y+z≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 6. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-x-y-a-b Fe.sub.a M.sub.x B.sub.y T.sub.b (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, T represents at least one element selected from Cu, Ag, Au, platinum group elements, Ni, Sn, Be, Mg, Ca, Sr and Ba, 0<a≦30, 2≦x≦15, 10<y≦25, 0<b≦10, and 12<x+y+b≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 7. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-x-y-z-b M.sub.x B.sub.y X.sub.z T.sub.b (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, X represents at least one element selected from Si, Ge, P, Ga, Al and N, T represents at least one element selected from Cu, Ag, Au, platinum group elements, Ni, Sn, Be, Mg, Ca, Sr and Ba, 2≦x≦15, 10<y≦25, 0<z≦10, 0<b≦10, and 12<x+y+z+b≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 8. A magnetic alloy with ultrafine crystal grains having a composition represented by the general formula:

    Co.sub.100-x-y-z-a-b Fe.sub.a M.sub.x B.sub.y X.sub.z T.sub.b (atomic %)

wherein M represents at least one element selected from Ti, Zr, Hf, V, Nb, Mo, Ta, Cr, W and Mn, X represents at least one element selected from Si, Ge, P, Ga, Al and N, T represents at least one element selected from Cu, Ag, Au, platinum group elements, Ni, Sn, Be, Mg, Ca, Sr and Ba, 0<a≦30, 2≦x≦15, 10<y≦25, 0<z≦10, 0<b≦10, and 12<x+y+z+b≦35, at least 50% of the alloy structure being occupied by crystal grains having an average grain size of 200 Å or less.
 9. The magnetic alloy with ultrafine crystal grains according to claim 1, wherein the balance of said alloy structure is composed of an amorphous phase.
 10. The magnetic alloy with ultrafine crystal grains according to claim 2, wherein the balance of said alloy structure is composed of an amorphous phase.
 11. The magnetic alloy with ultrafine crystal grains according to claim 3, wherein the balance of said alloy structure is composed of an amorphous phase.
 12. The magnetic alloy with ultrafine crystal grains according to claim 1, wherein said alloy is substantially composed of a crystalline phase.
 13. The magnetic alloy with ultrafine crystal grains according to claim 2, wherein said alloy is substantially composed of a crystalline phase.
 14. The magnetic alloy with ultrafine crystal grains according to claim 3, wherein said alloy is substantially composed of a crystalline phase.
 15. The magnetic alloy according to claim 1, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 16. The magnetic alloy according to claim 2, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 17. The magnetic alloy according to claim 3, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 18. The magnetic alloy according to claim 4, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 19. The magnetic alloy according to claim 5, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 20. The magnetic alloy according to claim 6, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 21. The magnetic alloy according to claim 7, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 22. The magnetic alloy according to claim 8, prepared by:(a) forming an alloy melt of the elements constituting the magnetic alloy; (b) liquid quenching the alloy melt to form an amorphous alloy; and (c) heat-treating the amorphous alloy at a temperature of from 450°-650° C. to cause crystallization.
 23. The magnetic alloy according to claim 15, wherein said heat-treating is conducted in a magnetic field. 